What two speed measurements do two observers in relative motion always agree on? In my opinion, one will be relative velocity...but can't figure out the other..? Any ideas? Also, photons of light have zero mass. How is it possible that they momentum? Cuz Momentum=mv....so if m is zero then how come they have momentum?...I thought abt this and researched...I think the reason is that photons don't have particle properties...kindly elaborate why exactly?